Quote Originally Posted by Burnt Toast
Nope, thats incorrect. When a particular drug is negative on the initial screen, it does not go thru confirmation. It would be a huge waste of money for both the client and the lab to do so.
Therefore, the confirmation GC/MS is only used on the drugs that tested positive on the initial screen.
I asked this a few weeks ago and did not get any replies...

I am not doubting you but do you know this for fact? I run urine samples/drug tests but I do run GC/MS samples all the time (surface waters). When you do a GC/MS run the resulting chromatogram will show everything that is in the sample at its cooresponding weight.... Its not more expensive and the machine will quantitate the peak areas automatically. For example, lets say the targeted THC metabolite peak is at 300 m/z and adderall metabolite peak is at 180 m/z....there are going to be peaks.

However, I would imagine a few scenarios where it could be possible 1) depending on what you fail for, there may be certain calibration ranges. For example if you only fail for THC then maybe a more narrow calibration range around the weight of those peaks is run? On the other hand, from a financial perspective I would think they would have one calibration to just scan for everything....it would be faster and more "streamlined" which is what these labs like quest diagnostics and Labcorp are all about....streamlined laboratory processes to turn a profit and run as many samples as possible.

2) Immunoassays are super specific, GC/MS is not. There may be a bunch of metabolites in in the GC/MS sample but only one specific peak is being quantitated and the quantitation limit is 15 ng/mg which may be about the same as 50 ng/mg.

The reason they use GC/MS is because it is a quantitative measurment and colorimetric assays are not quantitative (i.e., the concentration from a faint line does NOT mean there is more THC than a darker line). It may but its bad science to assume so.

Anyways...id like to pick somone's brain that has run GC/MS samples in a drug lab...